Process and apparatus for continuous digestion of fibrous materials



July 5, 1949.

C. F. C. RICHTER PROCESS AND APPARATUS FOR CONTINUOUS DIGESTION 0FFIBROUS MATERIALS Filed D96. 4, 1943 rro c INVENTCR: Johan ChrisfofferFredrik 0 rl Richter A fforney Patented July 5, 1949 PROCESS ANDAPPARATUS FOR CONTINU- OUS DIGESTION OF FIBROUS MATERIALS JohanChristoi'ier Fredrik Carl Richter, Oslo,

Norway, assignor to Aktiebolaget Kamyr, Karlstad, Sweden, a company ofSweden Application December 4, 1943, Serial No. 512,978 In NorwayOctober 15, 1942 Section 1, Public Law 690, August 8, 1946 Patentexpires October 15, 1962 2 Claims. 1

Digestion of wood or similar fibrous materials to cellulose, or forconverting wood into sugar, is effected at an increased pressure and atan elevated temperature, 130 to 180 C. or higher, according as thedigestion is carried out according to the bisulphite process, thesulphate process, the sulphurous acid process, or some other process. Inpractice, processes of digestion of this kind are exclusively carriedout periodically, that is to say, the digestion is effected batch-wise,a digester being filled with the raw material, such as wood chips, afterwhich the filling opening is closed and the digester liquor is admitted,and the temperature and pressure are then run up" in accordance withcurves gained from experience, the pressure being determined by thepressure of the steam plus the partial pressure of the gases developed.During the running up and digestion period the cock is usually "gassedoil," either in order to keep the pressure down, by discharge of air and$02, or in order to recover certain by-products, such as turpentine.

Many attempts have been made to solve the problem of continuousdigestion which would consist in continuously feeding the raw material,such-as wood chips, into one end of the apparatus used for thedigestion, and causing such raw material to move through said apparatuswhile admitting the digester liquor, steam, and so forth, and finallydischarging the material from the other end of the apparatus in the formof pulp, that is to say, a completely digested product. Methods andapparatuses of the most various kinds have been proposed and tested, butso far no fully tried-out and generally accepted solution of the problemhas come forth which has been put in practice on a commercial scale.

Among the problems which must be solved in order to eiiect a continuousdigestion, the following may be noted:

A. The continuous feeding of the raw material, such as wood chips, intothe apparatus against a high pressure.

B. A satisfactory impregnation of the raw material with the digesterliquor in conjunction with heating of the raw material as well as of thedigester liquor.

C. Feeding of the raw material through the apparatus in such manner thatcontrol is had of the movement of the mass and of the digester liquorwith relation not only to the apparatus but also with relation one tothe other, and in such manner that full control is had of the disotherwords, of the process of digestion itself.

D. The discharge 0! the digested-product.

Heretofore practically all proposed solutions of the problem were basedon the same basic principle as that applied to ordinary periodicdigesters, namely that the pressure in the digester is determined by thesteam pressure corresponding to the digesting temperature plus thepartial pressure of the gases present.

It has been found in practice that if the temperature of the digesterliquor is at the vaporizing point, the liquor has a pronounced tendencyto "travel" relatively to the mass towards the discharge point. Thisphenomenon is well known from the blowing" of ordinary digester-s. wherethere is risk of the liquor "going first, while the mass or pulp remainsbehind and must be washed out of the digester or pried out with the useof crowbars or the like.

This phenomenon is the reason why practically all attempts hitherto madeof performing a continuous digestion have failed.

The abovementioned diiliculty can be avoided by feeding the wood chipsinto the bottom of the digester and discharging the digested pulp andthe liquor over a discharge lip at the top of the digester, so that thedigester always remains filled up to its top.

But this solution of the problem suffers from certain disadvantages, andit is desirable, therefore, to solve the problem in such mannerthat thematerial is charged into the top of a vertical digester and thecompleted product is discharged from the bottom of the digester. Thiswill be evident from the following considerations upon which the presentinvention is based:

1. For practical reasons the digester should be vertical with its inletat the top and the discharge at the bottom. In the first place, thematerial or mass which is specifically heavier, is most easily feddownwards. In the second place, the feeding apparatus when placed at thetop of the digester, is more easily accessible for inspection andrepair, than if it is placed at the bottom of the digester. In the thirdplace, the digester is most easily emptied if the discharge is eilectedfrom the bottom of the same.

2. In accordance with the experience gained from periodic digestion, thematerial should pass through zones having diflerent temperatures,substantially in such manner that the material first passes through acomparatively cold zone for the purpose of impregnation, and thenthrough the digestion zone proper at full digestion temperature, andfinally through a somewhat colder discharge zone. Gas which is possiblyseparated a,474,se2

during the heating period in the digestion zone, such as S: in thebisulphite process, will attempt to move upward. and if the mass movesdownward, the gas will flow in counter-current to the material, which isthus subjected to a gradual preheating. Simultaneously the upwardflowing gas. such as $02, will successively encounter gradually coldermaterial. so that the gas is again dissolved and thus aids to effect theimpregnation.

3. The supply of chemicals-boiling liquorshould be eflected exclusivelywith regard to the predetermined quantity and strength, quiteirrespective of the pressure in the digester.

4. The movement of the material and of the digester liquor through thedigester should be positively controlled and must not be dependent uponaccidents or chances. In other words, and with reference to thephenomenon above mentioned, this means that the flow of the liquor mustbe brought under absolute control and that its tendency to "travel"relatively to the material must be eliminated. The fiow of the liquorthrough the digester must be determined exclusively by the quantity ofliquor introduced in the digester.

From the considerations above stated it is evident that a primarycondition for the process proceeding according to the lines abovestated, is that the digester must be completely dead, that is to say.completely filled with liquor, without any steam space at the top. Ifthe liquor is at the vaporization temperature, the steam pressureconstitutes the force that causes the tendency of the liquor to travel."This cause must be eliminated in the first place.

The conclusion is that the digester must be maintained undercompression, that is to say, the liquor therein must be under a pressurewhich exceeds the steam pressure of the digester liquor at the maximumdigestion temperature reached. This pressure of the liquor mayconveniently be identical to a pump pressure which is determined by theaid of a pressure regulator.

It will easily be understood that the digester being maintained undercompression by the aid of a pump pressure, the condition above statedunder point 2 can also be fulfilled, since in such case there is noobstacle to maintaining different temperatures in diiferent zones, aslong as the vaporization temperature at the prevailing pressure is notreached at any point. There is thus also nothing to prevent a coolerimpregnating zone being maintained at the top of the digester and ahotter digesting zone at a lower level, in accordance with point 2.

It will be understood from point 3 that the treating liquor itself mustnot be used as a compression liquid. This is for the reason that thefirst-named liquor must be controlled in such manner thata predeterminedquantity thereof is introduced into the digester, but the last-namedliquid must be so controlled that a predetermined pressure ismaintained.

It will be understood from point 4 that the compression liquid must notbe introduced in the digester at such a point that a variation of thequantity introduced influences the course of the process of digestion.The conclusion is that the compression liquid should be introduced atthe bottom of the digester in conjunction with the discharge, where thesaid liquid has no influence whatever on the course of the digestionitself, whether qualitatively or. quantitatively.

The quantity of compression liquid introduced, which in itself is ofsubordinate importance, be-

.4 comes automatically equal'to the difference between the quantity ofthe entire discharge and the sum of the quantities charged into thedigester of raw material, liquor, including chemicals, and steampossibly introduced.

By varying the discharge quantity in a suitable manner it is thuspossible to introduce a suitable quantity of compression liquid.

As regards the compression liquid itself, the only possibility thatpresents itself in practice is to use waste liquor, or so-called blackliquor, in order not to lower the content of chemicals and dissolvedsubstance in the liquor that is conducted to the liquor recovery. Foreconomical reasons it is not possible to use water as compressionliquid.

In accordance with the above considerations, the process according tothe present invention mainly consists in maintaining the material andthe treating liquor during their movement through the digester at apressure which is higher than the vapor pressure of the treating liquorat the maximum temperature prevailing in the digester, by pumping acompression liquid into the digester independently of the charging ofraw material and treating liquor into said digester. As a compressionliquid, waste liquor or so-called black liquor may preferably be used.It is suitable to introduce the compression liquid at the discharge endof the digester, so that said liquid simultaneously serves to dilute andto effect discharge of the digested product.

The present process and an apparatus for performing the same areillustrated diagrammatically and by way of example-in the accompanyingdrawing. v

Raw material is continuously supplied to the vertical digester 'I fromthe chip receptacle 2 which is provided with a vibration device 3 forproducing a uniform supply of chips to a measuring apparatus 4, forinstance in the form of a conveyer screw having a controllable speed, ablade wheel, a shaking apparatus, or some similar wellknown apparatus.Since the digester is assumed to be under a pressure which is higherthan the vapor pressure of the digester liquor at the digestiontemperature used, the digester must be completely filled with liquor.The feeding of the raw material, such as wood chips, into thisliquorfilled space under pressure is illustrated in the drawing aseffected by means of a combination of a rotating cell wheel 5, a sieve 6in a sieve housing provided at the top of the digester and surrounding afeeding screw I, a circulation pump 8, and conduits connecting the cellwheel, the sieve housing, and the pump.

The feeding is effected in the following manner: An empty cell in thecell wheel is partly filled with chips at ii and continues to rotate inthe direction of the arrow, more liquor entering the cell at i2, forinstance by leakage from It. At It the chips are emptied from the cellby the circulating liquor which is pumped by the pump 8 from the sieve 6through the connection 8 to the cell in question, and is returned fromsaid cell to the inner side of the sieve 6 wherethe chips brought alongare fed downward into the digester by the screws '1, while the liquor isseparated through the sieve and is returned in circulation to the cellwheel. The cell continues to rotate after the for gassing off air, forinstance, in a well known manner.

While the chips or other raw material are fed into the digester at thetop thereof by means of the screw 1, the impregnation or digester liquormay be supplied, in a manner previously known in itself, either directlyto the digester at 28, or at 28a through the receptacle l6 and the pumpIi. Fresh liquor, S: or other chemicals, may also be supplied wholly orpartially at a later stage, for instance at 28b or at 28c, suitably inconnection with a local circulation of the liquor and heating of thesame by means of steam, as indicated at H. In the upper portion of thedigester the impregnation of the material takes place at a comparativelylow temperature, 60 to 120 C., after which heatis supplied at I] eitherdirectly in the'form of steam or indirectly by local circulation of theliquor through a heat exchanger in a manner previously known in itself,for which reason such supply of heat has not been illustrated in thedrawing. The digestion temperature may for pressure actuated valve 21,so that the desired temperature of 120 C. is maintained at the bottom ofthe digester. If, on the other hand, the

temperature at the bottom of the digester tends to rise above the saidtemperature of 120 C. due.

instance be 140 to 180 C., or even higher.

Gases which may be separated due to the heating, will rise upward incounter-current to the material fed into the digester, and will thusproduce a certain preheating in the impregnating zone, and, due to thedecreasing temperature, the gas will at the same time be dissolved againand take part in the impregnation.

In a similar manner further heat'may be supplied at "a, or if desired,the temperature may be decreased at this point.

The completely digested mass, that is, the mixture of fibres and wasteliquor, is discharged from the bottom of the digester, and steam andgases are then allowed to escape at IS in the usual manner, the liquorbeing separated at 20. while the ready fibre product is conducted awayat 2| to be further treated, and the surplus quantity of liquor isconducted at l9 to the liquor recovery. The discharge may be effected,for instance, by the aid of a scraper device 22 and a sluicing device 23the capacity of which may be controlled by the aid of an automatic relay2i.

A portion of the liquor separated at 20 is returned by means of the pump25 and at a sumcient pressure for being introduced at 26 into the bottomof the digester to serve the purpose of diluting the stock or pulp inconnection with the discharge of the same. By means of a valve 21 whichis inserted in the pump conduit, and which is actuated by the pressurein the digester, the quantity of the compression liquid introduced at 26is regulated in such manner that the desired compression is maintainedin the digester, and consequently, during continuous operation thequantity of liquid pumped in at 26 becomes equal to the differencebetween the discharge quantity at 23 and the quantities of raw material,digester liquor, and steam introduced at I3, 28, 28b, 28c, l1 and Na,the discharge quantity at 23 being, of course, greater than the sum ofthe last mentioned quantities.

By constructing the relay 24 controlling the sluicing device 23 as atemperature relay, the discharge may be caused to take place at adesired predetermined temperature. For instance, if the relay 24 isadjusted for the temperature 120 0., then discharge will take place aslong as, owing to the supply at 26 of compression liquid having atemperature of, say, 80 C., the temperature in the digester is decreasedfrom, say, 170 C. to 120 C. at the discharge, and during all this timethe supplied quantity of compression liquid is of for instance, to thetemperature of the compression liquid increasing, then the relay 24 willincrease the discharge rate at 23, and as a result hereof the relay 21will also admit more compressionliquid at 26, so that the desiredtemperature of C. is maintained at the bottom of the digester. Ifanother temperature, say C., is desired, then the temperature relay 24is adjusted for this temperature, and in such case a temperature of 30C. will be maintained at the bottom of the digester. The capacity of thesluicing device 23 shall of course be so large that said device isalways capable of discharging not only the quantities of raw material,digester liquor, and steam introduced at I3, 28, 28b, 280, l1, and I la,but also the compression liquid introduced at", irrespective of thequantity thereof, and the temperature of this compression liquid must,of course, always be lower than the discharge temperature.

By the simultaneous use of a valve actuated by the pressure in thedigester which controls the supply of compression liquid, and a sluicingdevice which is influenced by the temperature, the entire course ofdigestion may thus be automatically controlled and regulated.

The process and the apparatus above described may be used, in principle,regardless of whether the treatment is efiected with bisulphite,suiphate, SOz-water, or some other combination. The supply of chemicalsand the supply of heat may be effected in accordance with well knownmethods which, therefore, need not be further explained.

I claim:

1. The process of continuous digestion of flbrous materialsunderpressure, which consists in continuously introducing the rawmaterial into an impregnation zone in a vertical digester at the topthereof, said impregnation zone having a temperature substantially belowdigestion temperature, continuously introducing digester liquor intosaid impregnation zone in the digester at the top thereof tocontinuously maintain the digester completely filled with a mixture ofraw material and digester liquor, supplying heat to said mixture in adigestion zone below said impregnation zone in said digester, passingsaid mixture of raw ma terial and digester liquor as a continuousdownward flow through the digester from the top to the bottom thereof,continuously discharging the digested product from the bottom of saiddigester, maintaining said mixture of material and digester liquorduring such flow through the digester under a pressure higher than thevaporization pressure of the digester liquor at the maximum temperatureprevailing in the digester, thereby preventing generation of vapour fromsaid digester liquor, controlling such pressure by introducing 7 intothedigester in the vicinity oi the bottomand the sum of the quantitiescharged into the digester per unit of time, and controlling the rate ofdischarge from the diaester in dependence upon the temperature at thebottom of the digester, thereby eilecting regulation the quantity oicompmsion' liquid introduced in the digester whereby a predeterminedtemperature is maintained at the bottom of the digester.

2. Apparatus for continuous digestion of iibrous materials, comprisingin combination a vertical digester, means for continuously introducingraw material into said digester at the top end thereof, means forcontinuously introducing digester liquor into said digester at the topend thereof, means for heating the fibrous material and digester liquorin said digester intermediate the top and bottom ends thereof, means forcontinuously discharging digested product from the bottom end 01' saiddigester, a conduit connected to said digester in the vicinity of thebottom end thereof for introducing a compression liquid in the digesterat the bottom end, means in said conduit actuated by the pressure insaid digester for controlling the flow of compression liquid throughsaid conduit so as to maintain a predetermined pressure in saiddigester, a discharging apparatus having a variable capacity associatedwith the bottom end of digeeter ior continuously discharging digestedproduct thereirom,

and an adjustable temperature relay actuated by the temperature at thebottom end or the digester and operatively connected to said dischargeapperatus so as to control the operation oi said apparatus in responseto the temperature at the bottom end of the digester.

JOHAN CHRIBTOH'ER FREDRIK CARL RICHTER.

summons crrnn The following references are 01' record in the ille ofthis patent:

UNITED sums PATENTS Number Name Date 1,186,655 Freeman June 13, 19161,478,614 Morterud Dec. 25, 1923 1,505,934 Olier Aug. 19, 1924 1,679,336Dunbar July 31, 1928 1,776,761 Morterud Sept. 23, 1930 1,856,453 BacchusMay 3, 1932 1,903,962 Dreyfus Apr. 18, 1933 1,918,181 Cram July 11, 19331,933,017 Jansa Oct. 31, 1933 1,954,012 Lemberger Apr. 10, 19341,980,390 Dreyfus Nov. 13, 1934 2,008,635 Brubacher July 16, 19352,200,034 Merrill May 7, 1940 FOREIGN PATENTS Number Country Date-519,793 France June 15, 1921 150,782 Great Britain Sept. 6, 1920 63,460Norway Apr. 28, 1941

